1. Field of the Invention
The present invention relates to a method for pressure compensation in the reversible hydraulic motors used to drive hoisting and closing cables of a cable operated crane, where the crane has, for example, a grab for picking up bulk material.
2. Description of the Related Art
In order to transfer bulk materials, grabs are often used, i.e., conveying vessels that can penetrate into a bulk or batch material, grasp it, pick it up, and empty it again above a discharge point. The grabs can consist, for example, of shovels articulated on a common crossmember that are placed, open, onto the batch material. As a result of the deadweight of the grab during closure the shovels penetrate into the batch material and are at the same time filled. There are different types of grabs, depending on the way in which closing of the shovels is performed.
Conventional cranes of this type use two cable operated grabs, in which two cables or pairs of cables, operating independently from one another, bring about the movement of the shovels. When pairs of cables are used the system is referred to as a four-cable grab. This type of grab requires a special grab winch with two drums which are driven predominately by means of reversible hydraulic motors.
Two-cable or four-cable grabs are operated by means of the two cables or pairs of cables. The open grab is placed onto the material to be grabbed with the closing cable slack. When the slack of the closing cables is taken up, the shovels are closed as a result of the kinematics of the grab. In order to cause the grab to penetrate under the effect of its deadweight into the batch material, the holding cable must from the beginning, be made slack or eased down during the closing operation. After the shovels are closed, the gripper is raised, slack in the closing cables is further taken up, and the holding cable must be made taut. This presents a problem during closing of the grab in the bulk material, in that the holding cables have a slack cable, specifically with cable lengths which are different during each hoist. At the same time, the closing cables are highly taut due to the closing operation. This results in pronounced pressure differences between the working circuits of the two hydraulic motors used for the hoisting cable or pair of hoisting cables and the closing cables or pair of closing cables.
If pressure compensation is cut after closing the grab shovels to reverse the hoisting motors, then the hydraulic motor for the closing cable would first be drawn through in the wrong direction. As a result, cavitation phenomena occurs in the hydraulic motor when the pressure in the working circuit of the hydraulic motor for the closing cable is switched over to the hydraulic motor for the holding cable. Frequent cavitations in the system cause damage to the hydraulic motors which have to be switched on and off repeatedly or changed over during the operation of the grab crane.
The object of the present invention is to provide a method and a device for pressure compensation in reversible hydraulic motors used for driving the hoisting and closing cables of a generic cable operated crane. More particularly, it is an object of the present invention to provide a method and device for eliminating damage to hydraulic motors that occurs due to cavitation.
In order to achieve the object according to the present invention, the working pressure in the working circuit of each hydraulic motor is detected and is adjusted via a control while a closing movement of the grab is being initiated so that by the end of the operation of closing the grab, identical working pressures exist on each hydraulic motor.
The present invention affords a solution which from the outset prevents pronounced pressure differences on the two hydraulic motors assigned to the different cables. As a result, the undesirable cavitation phenomena cannot occur in the motors. Instead, as early as during the operation of closing the grab, the working pressure detected in the working circuit of each hydraulic motor is used to bring about pressure compensation via a control, so that, at the end of the closing operation, compensation of the working pressures has taken place. When the hydraulic motors are then reversed in order then to raise the grab after the closing operation has been performed further pressure compensation is no longer necessary.
A device for performing the method according to the present invention includes a sensor inserted into each delivery line leading to the respective hydraulic motors for detecting the working pressure prevailing in the line. The sensors are connected to a control unit in order to process the signals representing the detected working pressure. The control unit is connected to a control block which compensates the working pressures in the delivery lines leading to their respective hydraulic motors.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawing. It is to be understood, however, that the drawing is designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawing is not necessarily drawn to scale and that, unless otherwise indicated, is merely intended to conceptually illustrate the structures and procedures described herein.